A new apparatus for the performance of automated batch light scattering measurements has been developed, with special application for the characterization of the concentration dependence of scattering intensity in highly concentrated protein solutions. It is extremely simple in design and preliminary experiments indicate that reliable measurements may be obtained at protein concentrations of up to at least 100 g/L. Using this system one can obtain a complete profile of scattering intensity over the entire range of concentration in a single experiment lasting less than 30 minutes (Fernandez, Minton).[unreadable] [unreadable] Sedimentation equilibrium experiments carried out on solutions of immunoglobulin G (IgG) at pH 7.4 in 150 mM NaCl, at concentrations of up to 125 g/L have been interpreted in the context of a model incorporating both attractive and repulsive interactions between IgG molecules. According to this model, IgG reversibly self-associates weakly to form predominantly trimers that constitute a significant fraction of total protein only at total concentrations exceeding 30 g/L (Jimenez, Rivas, Minton).[unreadable] [unreadable] A new description of the osmotic pressure of mixtures of nondiffusible solutes at arbitrary concentration has been developed. Data from the literature on the concentration dependence of the osmotic pressure of solutions of IgG at concentrations up to over 400 g/L was analyzed in the context of a model taking into account both attractive and repulsive interactions between IgG molecules, and the data may be quantitatively fit by the same model that describes the concentration dependence of the sedimentation equilibrium data (Minton).